As is well known, chlorine-containing polymers, especially polyvinyl chloride, suffer degradation from the effects of heat, for example during processing, which causes discolorations and an impairment of the mechanical properties. Therefore, stabilizers must be added to such polymers before processing. Essentially organotin compounds, organic and inorganic lead salts, organic antimony compounds or combinations of cadmium and barium carboxylates or of zinc and calcium carboxylates are used as stabilizers for polyvinyl chloride. Costabilizers are often added to these so-called primary stabilizers to improve their effectiveness. The mode of operation of primary or costabilizers, or their interaction (synergism) is described in the relevant literature, for example in the publication of L.I. Nass, "Heat Stabilizers" in Kirk Othmer, Encyclopedia of Chemical Technology, Vol. 12, 3rd Edition, page 225, John Wiley and Sons, 1980.
The effectiveness of the described stabilizers is sufficient for most applications; however, their use is subject to some important limitations. Due to their toxicity, lead, antimony, and cadmium stabilizers cannot be used in all fields, especially not in packaging materials for foodstuffs or in the medical field. Organotin compounds, some of which are toxically safe, are not suitable for a broad range of applications since they are relatively expensive and not readily available.
On the other hand, only an insufficient stabilization can be achieved with a physiologically unobjectionable stabilizer combination consisting of zinc and calcium carboxylates, as they are described in U.S. Pat. No. 2,446,976 (see Table A, Example 2, below).
For this reason, costabilizers such as epoxy compounds, polyols, organic phosphites, substituted dihydropyridines, 1,3-diketones or combinations of these compounds are added to zinc and calcium carboxylates. All these stabilizer combinations are, however, still not sufficiently effective, since the long-term stability of molding compositions stabilized this way proves unsatisfactory and/or the initial color is mediocre or bad. Especially both the initial color and the transparency of polymer compounds are greatly impaired by the high calcium carboxylate content required for long-term stability.